At San Ysidro, Calif., thousands stream daily into the U.S. from Mexico. For Customs, stopping the flow of contraband is a constant battle. CNET Road Trip 2012 saw the tech that helps.

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Marijuana in gas tank

SAN YSIDRO, Calif.--It's probably the busiest border crossing in the world, and every day, mixed in with the thousands of law-abiding people coming into the U.S. here from Mexico, smugglers attempt to outfox the law and sneak in all kinds of contraband -- like 38 pounds of marijuana hidden in wrapped packages found inside the gas tank of a car on Monday.

From pot to meth, cocaine to hidden illegal immigrants, the list of things that are smuggled in is long. And that's before you even start talking about stopping terrorists from getting into the country.

For U.S. Customs and Border Protection (CBP) officers, stopping the illegal activity is a never-ending battle, even as they attempt to make passage as fast as possible for everyone else.

To help them do their jobs, they have a slew of different technologies at their disposal -- and some very low-tech tools as well. As part of CNET Road Trip 2012, reporter Daniel Terdiman stopped in at San Ysidro to see what CBP is throwing at the problem.

Radiological monitors

Although much of the illegal activity at the San Ysidro port of entry involves drug smuggling, Customs and Border Protection is also on the lookout for anyone bringing radiological weapons -- or illegal material -- into the country. These yellow portals, which all vehicles crossing into the U.S. here must pass through, quickly monitor for any such material, alerting officers if any is detected.

However, there are all kinds of legal medical materials that might trigger the system -- such as residual chemicals from chemotherapy or stress tests. So if a vehicle sets off an alarm, officers will conduct a secondary search to quickly try to determine what triggered it.

Marijuana in gas tank

On Monday, after a Customs and Border Protection officer noticed a driver behaving nervously, and a sniffing dog reacted to something in the man's car, officers conducted a search of the vehicle. Using imaging technology, they determined that something in the gas tank wasn't right. They found a panel giving access to the gas tank, and inside turned up 38.44 pounds of marijuana, seen here.

Secondary backscatter

If officers believe it's warranted, they will drive cars through a set of backscatter panels at secondary inspection that generate imagery of the sides, the top, and the underside of the vehicle, highlighting any anomalies.

Pre-primary search

Although most rigorous searches of vehicles takes place at secondary inspection, Customs and Border Patrol officers will sometimes decide they need to conduct a search immediately, as in this photograph, taken before the car being searched had even reached primary inspection.

Buster

This is a "buster," a small hand-held device carried by some CBP officers that allows them to inspect small areas of a vehicle for any kind of unexpected material thickness. Like a stud-finder, the device can help the officers locate things like thick metal or organic material that shouldn't be there.

Backscatter image

This image was taken at a U.S. Border Patrol checkpoint in Arizona using a backscatter machine. U.S. Customs and Border Protection uses similar technology to get imagery of vehicles at its ports of entry, including the one at San Ysidro.

Camera

This camera, which is installed in the pre-primary area, recognizes cars' license plate numbers so that by the time a vehicle reaches an officer in a booth, that officer already has received information from law enforcement databases about the vehicle.

Booth

A primary inspection booth, where a CBP officer sits and processes people coming through the port of entry from Mexico into the United States. The officer will get information about each vehicle before it pulls up, and often has information about the driver -- or passenger(s) -- as well, allowing him or her to process each vehicle faster.

Annunciator

This device, which is installed in each primary inspection booth, is called an annunciator. It provides an alert when a radiological monitor mounted in the pre-inspection area of each lane detects something on an incoming vehicle.

RFID reader

This is an RFID reader that can automatically read several different kind of official identity documents, including U.S. Passport cards (but not Passport books); Border-crossing cards that allow non-citizens to travel up to 25 miles into the U.S. for up to 30 days; and SENTRI cards, which expedite passage for regular border crossers who have voluntarily gone through a background check and interview.

This helps speed up passage for everyone because the card reader gives the primary inspection officer information on the person using the card before they get to the booth.

Welcome to the U.S.

Lanes with open gates

Incoming vehicles are segmented into different lanes depending on the category of traveler. Those carrying SENTRI cards can choose from several lanes -- which move much faster than normal lanes. But officers can use these gates to redirect traffic depending on how many people in each category are coming through.